Painless versus conventional photodynamic therapy for treatment of actinic keratosis: Comparison of cell death and immune response in a murine model.

Aminolevulinic acid based photodynamic therapy (ALA-PDT) is a popular and efficacious treatment for actinic keratosis (AK). However, standard PDT can elicit stinging pain during illumination, and hence is not always favored by patients. In a new regimen called metronomic PDT (mPDT), similar to daylight PDT but using blue light, the illumination is delivered concurrently with ALA application rather than after a 1-hour pre-incubation (conventional regimen, cPDT). In the clinic, mPDT is not only painless but also nearly as effective as cPDT for AK lesion clearance. In this investigation, a murine AK model (generated by repeated UVB exposure) was treated with either mPDT or cPDT. Lesion clearance was followed by area measurement, and samples were harvested for mechanistic analyses. Compared to pretreatment (100%), the average lesion area was reduced to 47% and 32% in cPDT, and to 57% and 40% in mPDT at 1- and 2-weeks post PDT, respectively. Relative to untreated controls, enhanced cell death (histomorphology by H&E staining and apoptosis by TUNEL assay), and generation of Reactive Oxygen Species (ROS; CM-H2DCFDA staining) were observed in both cPDT and mPDT samples. Activation of cleaved Caspase-3 was specifically observed only in cPDT samples. Immunomodulation by inflammatory cells was observed by enhanced infiltration/retention of neutrophils and macrophages in metronomic PDT samples. Our results suggest that metronomic PDT can be just as effective as conventional PDT for treatment of AK, but the mechanisms may be quite different.